In the context of the present invention, said indication of horizontal position is part, with an indication of height, of an indication of position that corresponds to the current position of the aircraft. This indication of position is determined repeatedly by an onboard positioning device, using information that is detected at repeated moments by an associated receiver which cooperates with a conventional satellite positioning system, such as GPS, GALILEO, GLONASS, WARS, EGNOS systems, etc.
Although not exclusively, this horizontal position indication is more particularly used by a ground navigation aid system (see in particular FR-2 869 123) or a ground navigation/guidance system which are fitted on an aircraft and which are designed to improve the safety of the aircraft when taxiing on an airfield, in particular with the aim of avoiding collisions on the ground, runway incursions or navigation errors.
Yet the position indications coming from such an onboard positioning device are subject to errors that are caused by interfering phenomena such as a multipath phenomenon, essentially during a situation that is static or with low aircraft speed.
A multipath is a common phenomenon in the field of radio navigation and radio communication, which involves radiofrequency signals and which is a transient phenomenon depending on many factors.
Specifically, the signal transmitted by a marker or a satellite in the direction of a mobile user may be reflected by obstacles that it might encounter, such as buildings, the ground, or other vehicles. The power of the reflected signal and its phase depend on the distance from the reflector in relation to the onboard receiver antenna, on its hardware components, and on the power of the initial signal. Furthermore, the conditions for appearance and the impact that multipaths may have on a user depend strongly on the speed. For example, for a mobile user, knowing that the obstacle is static and that the satellite is moving, there are high probabilities that the reflected signals will hardly affect the position error, in any event over a short period. Conversely, when the user and the receiver are static, as in the airport navigation considered in the present invention, the position error may be significant.
With regard to airport navigation systems, A-SMGCS (Advanced Surface Movement Guidance and Control System) control and guidance systems or OANS (Onboard Airport Navigation System) systems are known. These airport navigation systems mostly use radio navigation means based on GNSS (Global Navigation Satellite System) satellite technology, such as GPS, GLONASS, or in the future GALILEO, and their SEAS (Satellite Based Augmentation System) augmentations, such as WAAS or EGNOS, or GEAS (Ground Based Augmentation System) augmentation.
It is known that to correct the errors inherent in GNSS satellite positioning systems, and in particular their sensitivity to multipaths (or to masking) or alternatively to the loss of a satellite, airport navigation systems rely on an estimation technique that uses an independent sensor to fill in the holes in the GNSS measurements. This estimation technique often uses inertial systems, speed sensors (tachometers) or distance sensors (odometer) for identifying the path of the aircraft during a temporary absence of GNSS measurements.
Specifically, when the aircraft is static or taxiing at low speed, and in the event of masking or multipath, the onboard GNSS receiver may be subject to unacceptable measurement errors that lead to jumps in position of several meters. Such measurement errors can be seen, for example, on a navigation screen showing the aircraft on a map of the airport. However, the use of the aforementioned estimation technique in such a situation is difficult because when the aircraft is static the path, distance or speed measurements are impossible. It is possible to use the fact that the observed inertial speed is zero, but inertial systems are subject to errors in speed, including in static mode, which are inherent in these systems. When the aircraft is moving at low speed, the use of an odometer or a tachometer is no longer appropriate as they are imprecise and above all are of low resolution which, during small movements, may lead the computer to error by not detecting the movements. With respect to the speeds worked out by the positioning device using a GNSS positioning system, these are also sensitive, though to a lesser degree than the position, to multipaths.
In addition, when the aircraft is moving at a steady speed it may be subject to multipaths or masking which may cause it to suffer jumps or losses of position. Combination with an independent system such as an odometer, a tachometer, or an inertial system makes it possible to solve numerous problems. However, for some dynamics, taking account of the errors inherent in these systems, position errors may occur because the combination does not always make it possible to distinguish those due to a movement from those due to a measurement error linked with a multipath.
It should be noted that document FR 2 888 643 discloses a method and a device for determining the position on the ground of an aircraft on an airport.